Molecular hydrogen is widely considered as a convenient energy vector. Its combustion in a fuel cell generates electricity with high yield and without any pollutant exhaust, water being the sole reaction product. However, although hydrogen is one of the most abundant elements on earth, molecular hydrogen exists only as traces in the atmosphere and has to be produced through processes that require some energy input. Thus the economically viable production of H2 from renewable sources is a major concern to the scientific community. Electrolysis (reduction of proton into H2 by mean of electrical energy) or photoelectrolysis (reduction of proton into H2 by mean of light energy sometimes with additional electrical energy) is one way to produce H2 from water (or a convenient proton source). Reduction of protons is apparently a very simple reaction. Unfortunately, it progresses slowly on most electrodes except on noble metals such as platinum. Hence hydrogen evolution is generally not observed at potentials near equilibrium (−400 mV vs. SHE at pH 7 in water) but requires the application of an overpotential also called activation potential. The same occurs for hydrogen oxidation. This kinetic limitation thus significantly reduces the energetic yield during a complete formation/uptake cycle of hydrogen and is therefore economically limiting for most industrial applications.
Fuel cells are electrochemical devices that convert the energy of a fuel directly into electrical and thermal energy. Generally, a fuel cell consists of an anode and a cathode, separated by an electrolyte through which they are electrically connected. A fuel, usually hydrogen, is fed to the anode where it is oxidised with the help of an electrocatalyst. At the cathode, the reduction of an oxidant such as oxygen, and usually air, takes place. The electrochemical reactions at the electrodes produce an electrical current and therefore produce electrical energy. Usually, thermal energy is also produced in parallel and can be used to provide additional electricity or for other purposes.
Currently the most common electrochemical reaction which is performed in a fuel cell is the reaction between hydrogen and oxygen to produce water. Molecular hydrogen may be fed to the anode where it is oxidised, the electrons produced passing through an external circuit to the cathode where the oxidant is reduced. Ions flowing through the intermediate electrolyte maintain charge neutrality. Fuel cells can be adapted to use other fuels such as methanol, hydrazine or natural gas.
Water electrolysers are made as fuel cells but are operated the reverse, with the application of electrical power between the electrodes and supply with water. Hydrogen is generated at the cathode and oxygen is generated at the anode.
Various difficulties have prevented the commercial development of fuel cell and water electrolyser technologies. The first one is cost and, in particular, the cost of the electrocatalysts employed at both the cathode and the anode to facilitate the electrochemical interconversion of 2H++2e− into H2 in the most versatile technology based on proton exchange membrane such as Nafion ®. The most commonly used electrocatalyst is platinum. Platinum is a very efficient catalyst and enables high currents to be produced in the fuel cell. However, its cost is high, and platinum is of limited availability. Therefore the metal catalyst is a significant contributor to the expense of the fuel cell. A further difficulty is that platinum is available in limited quantities on earth and world supplies cannot be expected to last more than a few decades if the use of fuel cells was to be generalized, in cars notably (Gordon et al. Proc. Natl Acas Sci USA, 2006, 1209-1214). The active layers for either hydrogen electro-evolution or electro-oxidation contain nanoparticles of Pt or other noble metals coated onto a carbon material: Pt nanoparticle catalysts stem from mid 20th century research on chemical catalysis in the gas phase and have been largely optimized up to now. While this solution is today the sole which is economically viable, nanoparticles (usually with a diameter of 5 nm and more) present no more than 10% of their atoms on their surface, while 90% of metal weight is unnecessarily immobilised, resulting in a cost/efficiency unfavourable balance.
Another difficulty with platinum comes from the fact that it is irreversibly inactivated in the presence of carbon monoxide. Many sources of hydrogen gas contain carbon monoxide impurities. The use of platinum catalysts therefore requires hydrogen fuel of high purity, with extremely low carbon monoxide levels. This adds to the cost of operating the fuel cell.
As a consequence, the existence of a global hydrogen economy appears to be dependent upon the development of new base-metal catalysts for hydrogen production and uptake.
Alternatives to platinum catalysts have been the object of numerous studies: one possibility is the use of hydrogenase enzymes at the anode. Electrodes based on the combination of hydrogenase enzymes with a carbon material have been disclosed in WO 2003/019705 ; US 2007/0248845 ; EP1939961 ; WO2004/114494, and more extensively in J. A. Cracknell et al., Chem. Rev. 2008, 108, 2439-2461. However, hydrogenases have been found to be highly sensitive to the presence of oxygen, and become inactive over a period of time when used in a standard fuel cell operating with oxygen (or an oxygen containing material such as air) as the oxidant. Moreover, they are very difficult to produce in a catalytically active form in significant amounts. A representative preparation requires two weeks for a few milligrams of enzymes corresponding to a few amount of active molecule since the molecular weight of the catalyst is about 55 kg.mol−1.
US2006/0093885 discloses functionalized carbon materials which can be used in electrochemical assemblies for electrochemical cells or fuel cells. But the catalyst is based on noble metal particles dispersed in a polymer.
WO2006063992 and US2006/0058500 disclose metal particles formed from a polymeric resin, a transition metal and a reducing agent and their use to make electrodes for fuel cells.
WO2006074829 discloses metal-organic complexes which are used as a coating on an anion conducting membrane, further reduced to form metal particle and fuel cells made from such membranes.
In these three last documents, metal ions are reduced to form nanoparticles and catalytic activity relies on the use of these nanoparticles.
The immobilization of metal-organic complexes on the surface of electrodes has also been achieved with the same aim: Kellet, R. M. and Spiro, T. G., (Inorg. Chem. 1985, 24, 2378) reported a series of cobalt porphyrins with high activity for H2 production, associated with low overvoltage in neutral aqueous solution. However, these compounds proved difficult to handle when covalently grafted to an electrode via an amide link with surface carboxylic acid groups: film instability or disrupting processes at either film-electrode or film-electrolyte interfaces were postulated. In another example, incorporating positively charged cobalt porphyrin complexes into a Nafion ® membrane coated on a glassy carbon electrode resulted in low electroactivity, reflecting the poor electron-transfer characteristics of Nafion ® films. T. Abe et al. (Polym. Adv. Technol. 1998, 9, 559) reported that cobalt tetraphenylporphyrin incorporated in a Nafion ® membrane coated on a bare pyrolytic graphite electrode can reduce protons only with a larger overvoltage (−0.7 V vs Ag/AgCl) in a pH 1 aqueous solution together with a considerably lower 70 h−1 turnover frequency value. Better turnover frequency (2.105 h−1) was observed but still with a large overvoltage at a potential of −0.90 vs Ag/AgCl and pH=1 for a cobalt phthalocyanine incorporated in a poly(4-vinylpyridine-co-styrene) film coated on a graphite electrode. In that case again the catalytic proton reduction was limited by the electron transfer within the matrix (Zhao et al. J. Mol. Catal. A 1999, 145, 245). Electropolymerization of [Cp*Rh(L)Cl](BF4) (L=bis-4,4′-bispyrrol-1-ylmethypmethoxycarbonyl]-2,2′-bipyridyl) leads to a stable film capable of proton electroreduction. Quantitative current efficiency corresponding to 353 turnovers was observed during a 14 hours electrolysis experiment at pH 1 using a carbon-felt electrode coated with the electropolymerized rhodium complex. Here again, turnover frequency is low and overvoltage dominates (Cosnier et al. J. Chem. Soc. Chem. Comm. 1989, 1259). More recently, the immobilization of diiron complexes (also known to be molecular electrocatalysts for hydrogen evolution) on carbon materials, either via the eleetropolymerisation of diazonium salts on glassy carbon (V. Vijaikanth et al. Electrochemistry communication, 2005, 7, 427-430) or via polypyrrole coating (S. K. Ibrahim et al. Chem Commun 2007, 1535-1537) has been described. However, although diiron complexes in solution show promising. electrocatalytic activity, there is little or no activity retained after grafting.
Document EP-1 914 755 discloses nanoscale electrochemical cell arrays wherein each cell comprises a well. A wall of the well comprises at least one electrode. Molecules can be coupled to the electrode via a linker. Organometallic complexes covalently or ionically linked to an electrode are disclosed. However such grafting provides surfaces which retain little or no catalytic activity.
Document US2004/0202876 discloses photofunctional molecules consisting of porphyrins oligomers or polymers covalently grafted to a substrate. However such complexes retain little or no catalytic activity.
Today, there are no viable solutions to efficiently replace platinum as electrocatalyst at the anode of a hydrogen fuel cell.